Electronic faucets of the type contemplated herein are increasingly used in public restrooms and other commercial applications to help prevent the transmission of infectious organisms and to help reduce the waste of potable water due to callous or mischievous conduct by the users. These electronic faucets can be activated by a user without any physical contact and are typically designed to only permit water flow when a user or other object is detected at the faucet.
Such faucets are well known in the art. See, for example, U.S. Pat. No. 5,555,912 to Saadi et al., U.S. Pat. No. 5,224,509 to Tanaka et al., U.S. Pat. No. 4,767,922 to Stauffer, and U.S. Pat. No. 4,709,728 to Ying-Chung. As these patents demonstrate, active infra-red (IR) detectors in the form of photodiode pairs are commonly used in these faucets for object detection. Pulses of IR light are emitted by one diode with the other being used to detect reflections of the emitted light off an object in front of the faucet. Different designs utilize different locations on the spout for the photodiodes, including placing them at the head of the spout, as in the Saadi et al. and Tanaka et al. patents, or farther down the spout near its base, as in the Stauffer and Ying-Chung patents. Some have proposed placing the emitter and receiver at different locations, as in U.S. Pat. No. 5,549,273 to Aharon, while others have proposed IR transceivers that are entirely separate from the spout, as in U.S. Pat. No. 5,625,908 to Shaw and U.S. Pat. No. 5,577,660 to Hansen.
Apart from the location of the IR sensor elements, a number of other design considerations exist in the use of active IR sensors, including how the sensors will be oriented, where the control electronics will be located, and how the sensors will be utilized to make decisions regarding switching the faucet on and off. Generally, the orientation of the sensors determines their field of view. In most designs the sensors are oriented either horizontally (i.e., so that their optical axes are parallel to the bottom surface of the spout base) or downwardly (i.e., inclined downwards into the sink basin). A benefit of horizontally orienting the sensors is that a user's hands can be detected sooner than if the sensors are oriented downwardly. However, one problem with horizontal orientation is that upon the faucet switching on, the water stream may reflect the transmitted IR light, even when the object that triggered the faucet is no longer present. One technique for compensating for this reflected light is disclosed in U.S. Pat. No. 5,566,702 to Philipp. In the Phillip design, the amount of reflected IR light due to the water stream is determined and then, during normal use, this amount is subtracted from the signal received whenever the faucet is running. Faucets utilizing downwardly directed sensors do not typically have this same problem and can be designed so that no special processing of the reflected light is required to accommodate the water stream. See, for example, U.S. Pat. No. 4,894,874 to Wilson. However, as indicated above, these designs typically result in an undesirable characteristic; namely, that they do not detect a user's hands and start the water flow until the user's hands are directly underneath the faucet.
The active IR sensors are operated by a control circuit that activates the LED transmitter and then monitors the LED receiver for reflections of the infra-red light. In some instances, the control circuit is mounted within the spout itself, as in the Wilson patent and U.S. Pat. No. 4,872,485 to Laverty, Jr. In other cases, it is designed to be located with the valve or in some other location under the sink, such as in U.S. Pat. No. 4,823,414 to Piersimoni et al. and U.S. Pat. No. 4,604,764 to Enzo. Locating the control circuit within the spout itself can create complications that may result in an overly complex mounting scheme or in a mounting scheme in which the electronics remain accessible after installation of the faucet. For example, in the Wilson patent, the printed circuit board is screwed onto a base in an arrangement that takes up a considerable amount of the space within the spout and that is easily accessible even after installation. Such access may be undesirable in commercial applications where, once installed, the faucet may be subjected to mischievous tampering or vandalism.
One of the difficulties in providing a consistent operation in which the faucet switches on and off at the appropriate times is in designing a control circuit that can properly interpret the signals received from the IR sensor and that can adjust to abnormal circumstances and changes in ambient conditions. To this end, the control circuits are increasingly becoming microprocessor based circuits that utilize sophisticated algorithms to operate the IR sensors and interpret the received signals. Many of these algorithms are variants on the basic approach of comparing the received signals to a threshold value that represents a background reading of the reflected IR and, if the received signal is greater than the threshold, then the presence of an object is assumed and the water flow is switched on. See, for example, the above-noted patents to Philipp and Aharon, as well as U.S. Pat. No. 5,217,035 to Van Marcke. As shown in the Philipp patent, this comparison can be accomplished using an analog comparator that compares the received signal to a reference with the output of the comparator providing a binary input to the circuit 's microprocessor. The reference voltage can be generated through software by using an output of the microprocessor to charge the capacitor for a certain length of time and, therefore, to a certain votage.
These circuits may also include calibration routines that are used to initially determine the proper threshold or reference voltage and to periodically adjust for slow changes in ambient conditions. See, for example, the Philipp patent and U.S. Pat. No. 5,570,869 to Diaz et al. In the Philipp faucet, the IR sensor is periodically used to take a current background reading which is compared to a stored background reference level. The stored background level is then incrementally adjusted up or down depending upon whether the current background reading is more or less than the stored value. In the Diaz et al. faucet, a continuous calibration approach is used to calibrate to all detected changes, including those for which activation of the faucet is desired. As with the Philipp faucet, the Diaz et al. control circuit compares reflected IR pulses to a reference voltage and initiates water flow when the signal strength due to the reflected pulses exceeds the reference. However, the Diaz et al. circuit automatically adjusts the strength of the transmitted IR pulses so that the signal due to the reflected pulses is equal to the reference voltage. The received signal and reference are provided as inputs to a comparator whose output is used to increase the strength of the IR pulses when the received signal is less than the reference and to decrease the strength of the IR pulses when the received signal is greater than the reference. In lieu of adjusting the strength of the transmitted IR pulses, the circuit can adjust the reference voltage to track changes in reflected signal strength. Consequently, the control circuit attempts to calibrate to all detected changes, including those due to the presence of the water stream or other object. This can be disadvantageous because, rather than detecting a baseline or background level, the circuit tracks all changes and the comparator's reference voltage is therefore undesirably affected by received signals that indicate a detected object.
One problem common to most currently available electronic faucets is that their control algorithms assume that the presence of a user's hands under the faucet will always result in an increase in reflected IR light. However, when such faucets are used in conjunction with metal or other highly-reflective sink basins, the presence of a user's hands under the faucet may actually decrease the amount of reflected IR light. Accordingly, there exists a need for an electronic faucet that can be used in any installation without the need for special setup procedures to accommodate the characteristics of the environment in which the faucet is placed. There also exists a need for an electronic faucet that provides a simple and effective mounting scheme for the control circuit and that precludes access to the electronics once the faucet has been installed.